Problem: What is the greatest common factor of $20$ and $30$ ? Another way to say this is: $\operatorname{gcf}(20, 30) = {?}$
Answer: The greatest common factor is the largest number that is a factor of both $20$ and $30$ The factors of $20$ are $1$ $2$ $4$ $5$ $10$ , and $20$ The factors of $30$ are $1$ $2$ $3$ $5$ $6$ $10$ $15$ , and $30$ Thus, the greatest common factor of $20$ and $30$ is $10$. $\operatorname{gcf}(20, 30) = 10$